1 post / 0 new

Adjusting bias in RD50 amps - try this


Chris Metcalfe (chris_m)
Username: chris_m

Registered: 10-2006
Posted on Thursday, November 02, 2006 - 06:09 am:   

In reply to myself in the thread above, I should have read the schematic properly!! What I did was:
-with schematic and meter, locate the 75k restrs r68 and r69, and check value
-clean the ends of the resistors, to ensure solder contact, with an emery board WITH POWER OFF and voltage drained!!
-solder two upright solid wires across the ends of r68 and r69, leaving them standing up AND CLEAR OF OTHER COMPONENTS. This is to avoid repeatedly re-soldering the ends of r68/69 as you experiment with values.
-bridge the 75k resistors with a parallel resistor, increasing voltage drawn from the 6L6 cathode. I found that bridging with 500k increased current draw from about 5ma to about 9ma
-experiment with resistor ranges 1meg -500k initially until you have the idle draw you need.
I would think that 15ma would be the upper limit, perhaps 12ma per tube a reasonable compromise.
Obviously DO NOT do this unless you are competent- if you can't understand any part of this post, or don't understand how to measure idle current draw in the RD50, DON'T attempt the modification. When measuring current draw, use one hand only, clipping the -ve meter lead to chassis away from the power leads.

This applies to the RD50 10 and 12 models ONLY - they are self-biasing, and this simply alters the self-biasing point setting. Don't try it on any other circuits, and don't set the idle current too high- although, if you understand this post, you'll know that already!!!

Hope this is useful
Dirk Wacker (doc_holiday)
Username: doc_holiday

Registered: 02-2007
Posted on Tuesday, February 13, 2007 - 03:17 am:   

Hi Chris, this sounds very interesting. I?m sure that I will not do this myself, but I have a very experienced amp-tech here in town who can do it for sure. The question is, is it really necessary and recommendable to perform this mod ? I want to put the TAD 6L6GC-STR matched power tubes into my 112RD50 and I?m looking for a strong and percussive clean tone for country and rockabilly stuff.